Conventionally, differently from a primary battery having no charge ability, a rechargeable secondary battery having charge and discharge characteristics is actively under study with the development of advanced technologies including digital cameras, cellular phones, laptop computers, hybrid cars, etc. Examples of the secondary battery include a nickel-cadmium battery, nickel-metal hydride battery, nickel-hydrogen battery, a lithium secondary battery, etc.
Of the above-mentioned secondary batteries, a lithium secondary battery has a driving voltage of 3.6V or more. The lithium secondary battery may be utilized as a power source for portable electronic appliances, or may be utilized in high-power hybrid cars when a plurality of lithium secondary batteries are connected in series. Since the lithium secondary battery has a higher driving voltage three times that of the nickel-cadmium battery or nickel-metal hydride battery and also, has superior energy density per unit weight, the use of the lithium secondary battery is rapidly increasing.
At present, a lithium ion battery has been fabricated, wherein a cathode and an anode, which are insulated by a separator interposed therebetween, are wound into a cylindrical or prismatic electrode assembly, and after the resulting electrode assembly is inserted into a metal can, an electrolyte is injected into the metal can. As the metal can is sealed, the fabrication of the lithium ion battery is completed.
More particularly, a conventional lithium ion battery includes a cathode in which a cathode active-material coating layer is provided on one surface or both surfaces of a cathode collector, and an anode in which an anode active-material coating layer is provided on one surface or both surfaces of an anode collector, the cathode and anode being wound with a plurality of separators interposed therebetween.
In the case where active-material coating layers are provided on both surfaces of an electrode collector, the active-material coating layer provided on one surface of the electrode collector is generally shorter than the active-material coating layer provided on the other surface of the electrode collector. Typically, it is desirable that a length and width of an anode be longer than a length and width of a cathode, to prevent extraction of lithium ions from the cathode.
Recently, a battery has been developed, which is changed in configuration such that active-material coating layers applied to both surfaces of a cathode collector are longitudinally deviated from each other, causing a part of one active-material coating layer so as not to be included in the other active-material coating layer.
FIG. 1 is a sectional view of a conventional battery having the above-described configuration, and FIG. 2 illustrates a “jelly-roll” configuration of the wound battery. Considering the configuration of the conventional battery in detail with reference to the drawings, the battery includes a cathode in which cathode active-material coating layers 20a and 20b are provided on at least one surface of a cathode collector 10, an anode in which anode active-material coating layers 40a and 40b are provided on at least one surface of an anode collector 30, and a plurality of separators 50a and 50b interposed between the cathode and the anode. At least one of a winding beginning portion and winding ending portion of the cathode collector 10 or anode collector 30 contains a cathode uncoated part 10′ or anode uncoated part 30′ where no electrode active-material coating layer is present. These uncoated parts 10′ and 30′ are provided with electrode leads 60 and 70 to be connected to exterior terminals. Both the electrode leads, i.e. cathode lead 60 and anode lead 70 are arranged in the same direction.
When the cathode active-material coating layer 20a comes into contact with the anode with the separator interposed therebetween, the cathode active-material coating layer 20a must overlap the facing anode active-material coating layer 40b (in other words, must have a smaller area than that of the anode active-material coating layer 40b), in consideration of a winding deviation and positional change caused upon charge and discharge of the battery. Under this condition, the boundary between the cathode active-material coating layer and the cathode uncoated part 10′ comes across the anode active-material coating layer 40b. This causes occurrence of micro-holes or shrinkage and damages to other functions of the facing separator 50, resulting in significant heat emission upon contact between the anode active-material coating layer 40b and the cathode uncoated part 10′.
As shown in FIG. 1, the cathode lead 60 faces the anode uncoated part 30′ with the separator 50b interposed therebetween and thus, there is a risk of short circuit between the cathode lead 60 and the anode uncoated part 30′ (see region A).
Further, as shown in FIG. 1, since the anode active-material coating layer 40a provided at an upper surface of the anode collector 30 (see region B) faces the boundary of the cathode active-material coating layer 20b (see region C) with the separator 50a interposed therebetween, and the anode active-material coating layer 40b provided at a lower surface of the anode collector 30 faces the boundary of the cathode active-material coating layer 20a (see the region B) with the separator 50b interposed therebetween, there is also a risk of short circuit. Meanwhile, when the anode and cathode active-material coating layer come into contact with each other under the occurrence of short circuit, there exist a negligible short circuit current and heat emission because of a high electric resistance of the cathode active-material coating layer. However, when the anode comes into contact with the cathode uncoated part (i.e. a part of the cathode collector where no cathode active-material coating layer is present), an insufficient electric resistance causes a serious short circuit current and heat emission which act as dangerous factors significantly deteriorating safety of the battery.
To solve the above-described problems, conventionally, a method for providing an insulator in facing region between the cathode uncoated part and the anode has been adopted.
FIG. 3 illustrates a configuration wherein insulators 90a, 90b, 90c and 90d are provided in the conventional battery shown in FIG. 1. Referring to FIG. 3 together with FIGS. 1 and 2, the anode lead 70 is attached to the anode uncoated part 30′ of the anode collector 30 where the anode active-material coating layers 40a and 40b are not present, and the anode lead 70 faces the cathode with seven layers of separators 50 interposed therebetween. Provision of the sufficient number of separators 50 eliminates a necessity for an insulator around the anode lead 70 facing the separators 50. Furthermore, a protective tape 80a is provided at an opposite side of the anode lead 70, eliminating a necessity for an insulator.
Note that a distal end of the winding ending portion of the anode faces, in either direction, the cathode with only one layer of the separator interposed therebetween and therefore, an insulator must be provided therebetween.
Also, the cathode lead 60 is attached to one side of the cathode uncoated part 10′ in the winding beginning portion of the cathode collector 10 where the cathode active-material coating layers 20a and 20b are not present, and although not shown in the drawing, the cathode lead 60 faces another cathode with four layers of separators 50 interposed therebetween, eliminating a necessity for an insulator. However, the other side of the cathode collector 10 opposite to the cathode lead 60 faces the anode with only one layer of separator 50 interposed therebetween, having a necessity for an insulator.
Note that both a beginning portion and ending portion of the cathode active-material coating layer face the anode with only one layer of the separator interposed therebetween and therefore, it is necessary to provide an insulator therebetween.